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Full text of "Handbook Of Chemical Engineering - I"

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MECHANICAL SEPARATION                             265
all the compartments and if one gate is given greater closure the strength of the currents increase in the others.
In the first compartment a feeble current is maintained just enough to float up the lighter impurities but not sufficient to prevent the small fine purified middlings from passing through the screen. The strength of the air current is progressively increased in the other compartments and by thus using graduated air currents the loss of middlings which would follow with an air current strong enough to remove all the impurit es is avoided. The device is provided with a brush for unblinding the screen cloth which moves in contact with the lower side of the screen cloth and is actuated by an endless chain. The purified middlings passing through the screens are removed by a worm in the bottom of the box. The bran and impure middling removed by the fan are recovered by cloth filtration. The capacity of these devices ranges from 6 to 30 bushels per hour. The area of screen cloth ranges from 7 to 20 sq. ft. The overall dimensions of the enclosing box or case range from 6 ft. height by 7 ft. length and 4 ft. width to 8 ft. height, 11 ft. length and 5 ft. width. The screens are shaken between 500 and 600 times per minute.
Mechanical arrangements similar to the middlings purifier have been employed on separation problems in a number of industries. They have met with some success, for example, in separating the woody part of the cork bark from the pithy.
Sutton, Steele & Steele Table.—Some of the separating problems involving slight differences of specific gravity or requiring merely a grading to effect the separation, for which this device has been advocated are: Grading of walnuts in the shell, peanuts in the shell and shelled and seeds of all kinds. The inventors state that in the grading of these they have noticed that the discard contains the most of the weevil germ. It is also recommended for cleaning cereals and rice and for separating cork. The machine was originally developed as a dry-ore concentrator and its arrangements resemble that of a Wilfley concentrating table. They consist of a flat deck mounted a short distance above the floor level by means of rocking legs. The deck is clothed with cloth and for treating coarse materials more open cloth is used than for fine materials.
A reciprocating motion is given to the deck by an ordinary eccentric and since the rocking legs incline towards the eccentric or head motion end of the machine, at each forward stroke the deck rises slightly and falls back an equal amount on the return giving the necessary differential for advancing feed along the deck. An ordinary centrifugal fan is built into the frame of the machine and a flexible connection leads to an air chest under the cloth mounted deck. Regulation of the air through the cloth is obtained by a gate on the suction side of the fan. A series of shallow wooden riffle strips are secured to the deck parallel to the line of the pulsions. Material is fed at the upper right-hand corner looking down the deck from the head-motion end. It spreads out over the deck under the combined influence of the advancing motion and the slightly transverse inclination given the deck. By interstitial action due to the shaking of the deck, explained elsewhere in this work, the small particles work down through the bed of material on the deck and advance to the lower end where they are discharged. The large particles which stay on top of the bed move under the influence of the transverse slope of the deck more nearly across the table and discharge over the long side. The deck is 10 ft. long and 5 ft, wide.
The riffles assist in guiding the small particles to the end of the deck or what is the same thing prevent them from working transversely across the deck with the large particle. The riffles are of such height that they do not interfere very much with the transverse travel of the large particles. If the small particles are of greater specific gravity than the large there will be separation according to gravity. If the